Gap junctions in rabbit corneal epithelium: Limited permeability and inhibition by cAMP

Rabbit corneas were mounted in Ussing chambers, and the apical membrane of the superficial cells (SCs) was permeabilized by exposure to digitonin in a Ca²⁺-free Ringer solution. This treatment resulted in the generation of large (60.7 ± 13.2 μA/cm², n = 25) Na⁺-dependent tear (T)-to-stroma (S) short-circuit currents (I(sc)). The I(sc) was abolished by ouabain and by 1.4 mM Ca²⁺ and was inhibited by heptanol, 18α-glycyrrhetinic acid, and dieldrin, effects consistent with the notion that corneal transepithelial fluxes include translocations through gap junctions (GJs) before basolateral membrane transport. T-to-S I(sc) were also generated when T-side Na⁺ was replaced by K⁺, eliciting a T-to-S K⁺ flux via basolateral K⁺ channels and when, with either Na⁺ or K⁺ on the T side, channels were introduced at the apical membrane with amphotericin B. The I(sc) in all four conditions exhibited similar sensitivity to GJ inhibitors and were inhibited by adenosine 3',5'-cyclic monophosphate (cAMP) elevation. Fluorophotometry combined with SC permeabilization with digitonin demonstrated that the half-time for the SC to sub-SC movement of 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (mol wt 540) exceeded 3 h. These results indicate that junctional communications along the epithelial stratification axis are highly restricted and modulated by cAMP concentration.